This invention relates to a control system for regulating the rate of fuel flow to the burners of a gas turbine engine having a rotary compressor whose discharge pressure is detected and is used in establishing the flow rate at various times such as when the engine is accelerating. More particularly, the invention relates to a system for controlling a turbine engine adapted to burn gaseous fuel such as natural gas supplied from a conventional source and normally maintained at a predetermined and comparatively high pressure such as by a boost compressor and a constant pressure regulator.
The rate of fuel flow to the engine is controlled by a fuel metering valve which is adapted to be opened and closed to increase and decrease the fuel flow. Opening and closing of the value is effected in response to a signal, usually a function of speed error, produced by a governor which regulates the fuel flow so as to keep the engine running properly under different operating conditions.
When the engine is to be accelerated, the governor may attempt to open the metering valve to its full open position and allow maximum flow of fuel to the engine. If the engine receives too much fuel for a given set of operating conditions, surge and/or dangerously high temperatures can result. To protect the engine during acceleration, an acceleration limit control detects an engine operating parameter (e.g., compressor discharge pressure) and prevents the metering valve from opening beyond a given limit position even though the governor may be calling for more fuel. The limit position of the valve varies as a function of compressor discharge pressure, and the valve is permitted to open to a wider limit position as compressor discharge pressure increases and the engine is capable of safely tolerating more fuel. Thus, the limit position of the valve is scheduled with respect to compressor discharge pressure, the schedule commonly being referred to as an acceleration limit schedule.
When the engine is operating on gaseous fuel, it is possible for the gas supply pressure to drop from its normal high value. This occurs, for example, if the boost compressor should malfunction or if the supply gas main pressure should drop inordinately. When the gas supply pressure to the metering valve falls, the rate of fuel flow to the engine is reduced when compared with the flow rate that prevails with the valve in the same position but with the gas supply pressure at its normal high value. When the normal acceleration limit schedule for normal supply pressure is maintained at lower supply pressures, the metering valve is prevented from opening sufficiently far to deliver enough fuel to enable the engine to accelerate at the maximum safe rate, or to develop optimum steady state power, at the lower supply pressure.